Modern ventilation systems, both for domestic use, public buildings and industrial premises, are frequently provide with heat exchange, or heat recovery systems. The basic principle for such systems is that heat is taken from outgoing air and used to preheat incoming air. A number of different design principles [add ref] are utilized for the heat exchange. According to one design principle heat batteries in the form of metal plates are alternately heated (collecting heat) and cooled (deliver heat) by being placed alternately in an outgoing air stream and an ingoing air stream, respectively. If the air streams are switched to a stationary heat battery, the exchange assembly is referred to as a switching heat exchanger. Another implementation of the same principle is the rotary heat exchanger, wherein the heat batteries are placed in a rotating arrangement moving the plates of the heat battery from the outgoing (heated) air stream to the incoming air stream (cool).
Large scale installations of ventilation systems with heat exchange facilities, such as those found in larger office buildings, public buildings and industries, often relies on heat exchange arrangement using a heat transfer fluid for transferring heat from the outgoing air stream to the incoming air stream. A prior art ventilation system utilizing a heat transfer fluid arrangement is schematically illustrated in FIG. 1. The ventilation system 100 comprises an incoming air duct 105 provided with a fan 110 for forcing air into the premises. An outgoing air duct 115 vents the air out of the premises with the aid of the fan 120. The thick arrows indicate the direction of the flow of air. The outgoing air duct 115 is provided with a heat collector unit 125, for example in the form of a radiator. The ingoing air duct 105 is provided with a heat delivery unit 130, preferably also in the form of a radiator. The heat collector unit 125 is connected to the heat delivery unit 130 with a tubing arrangement 135, forming the heat exchange system. The heat exchange system may in addition include one or more circulation pumps and expansion vessels etc. A heat transfer fluid is circulated in the heat exchange system (the narrow arrow indicate the flow of the heat transfer fluid). The heat of the outgoing air heats the heat transfer fluid in the heat collector unit 125 and the heat transfer fluid transfer the heat to the heat delivery unit 130, which warms the incoming air. The heat transfer fluid should have suitable thermodynamical properties for receiving and delivering heat as well as suitable fluidic properties. In most cases water is the most suitable heat transfer fluid. However, in certain application and in certain areas were will be a risk of the heat transfer fluid freezing in the heat delivery unit 130, wherein the heat transfer fluid is cooled down. This can be true in the tempered part of the world, wherein during a winter day the incoming air could be significantly below the freezing point of water. It should be noted that the freezing effect at the heat delivery unit 130 is not only dependent of the temperature of the incoming air, but also of the speed of the air flow, typically giving an effective freezing effect significantly lower than indicated by the temperature alone. Freezing of the heat transfer fluid leads to immediate malfunction of the heat exchange and possibly also causing shutdown of the entire ventilation system.
Freezing of the heat transfer fluid is inhibited by adding an anti freeze agent to the heat transfer fluid. Several anti freeze agent are known in the art and can be divided into two main groups: Anti freeze agents based on salt solutions, for example alkali salts, and anti freeze agents based on organic compounds, for example alcohol or glycol. Several anti freeze agents are known in the art and widely used for freezing inhibition in different kinds of application. A range of anti freeze agents are commercially available and sold under different brand names such as [add ref]. In table 1 a range of anti freeze agents and their properties are listed. The listed freezing points refer to different mixing proportions of the anti freeze agent and the water, and reflects the typical usage, wherein an operator has specified what freezing point is accepted by the heat exchange system and adds an amount of anti freeze agent to the heat transfer fluid to achieve the mixing proportions corresponding to the determined freezing point.
Table 1 illustrates the effectiveness regarding lowering the freezing point using these known anti freeze agent. Upon inspection another inherent property of the anti freeze agent is apparent, that the heat transfer capacity of the heat transfer fluid (is strongly adversely affected by the addition of anti freeze agent. Taking the common anti freeze agent polypropylene glycol as an example, mixing with water so that a freezing point of −10° C. is achieved results in a reduction of the heat transfer capacity with about 30% as compared to pure water. If polypropylene glycol was added in an amount to let the heat transfer fluid has a freezing point at −30° C. the reduction of heat transfer capacity will be in the order of 60%. The efficiency of the heat exchange system follows the heat transfer capacity of the heat fluid, and can never be better than that value.
In large scale installations the amount of freezing agent in the heat transfer fluid is typically decided on at installation and only changed at large maintenance operations. Typically the mixing proportions, often referred to as the level of anti freezing agent, is checked during regular maintenance and, if the level is found to be to low, anti freeze agent is added.
The above described scenario is problematic in an energy recovery perspective. The level of anti freeze agent is typically determined for a worst case scenario. In northern Scandinavia, for example, adapted to handle incoming air at a temperature of −30° C. or below, which depending on the anti freeze agent used, gives a decreased heat transfer capacity of 40-60%. Typically this low freezing point is only required a few days each year, even in northern Scandinavia. As the level of anti freeze agent is typically not changed the heat exchange system operates with the same low efficiency also then not needed due to the outside conditions. As this is the vast majority of the time, the losses in efficiency, measured on a yearly basis, are very large. Also in areas with less cooled winters, for example central Europe, wherein a heat exchange system typically should be designed for occasional freezing weather. Also in this case, with a heat transfer fluid with a freezing point of for example −8° C., the losses will be significant. Thus were is a problem of optimizing the level of anti freezing agent both to lower the freezing point to a sufficient temperature and at the same time keep the heat transfer capacity as high as possible.
A further problem arises from the fact that improper mixing of different anti freezing agents can lead to problems in analysing the level. The percentage of anti freezing agent in the heat exchange fluid is often measured by a simple refractive measurement, which gives a decent estimate. The measurement method is normally reliable, but if certain anti freeze agents are mixed, for example glycol based anti freeze agents of different kinds, the measurement may become unreliable. Typically the measurement is affected in the way that it indicates a lower level of anti freeze agents than the actual level. This will lead the operator to add even more anti freeze agent, typically resulting in a heat transfer fluid with a freezing temperature way lower than any conceivable temperature. It should be noted that from the perspective of the thermal properties of the heat transfer fluid, it is in many cases acceptable, or in some cases possibly even advantageous, to mix different anti freezing agents at least within the two basic categories. The problem arises from the effect on the measurement method. The problem is accentuated by that anti freeze agents are typically sold by their product name, and it is not evident for an operator of a ventilation system what the active substances are, nor their mixing properties.
A further problems comes from the fact that operators often with very limited knowledge of the drawbacks of reducing the heat transfer capacity, often adds significantly more anti freezing agent than recommended, just to be sure that the system will not freeze. This further reduces the efficiency of the heat exchange system.
The problem of having low heat transfer capacity due to a level of anti freeze agent that is unnecessary high most of the time is not limited to heat exchange systems for ventilation purposes. The same problems may occur in for example sun panel arrangements, greenhouse heating systems, systems for heating roads, airstrips and outdoor pedestrian areas.
Methods of separating anti freeze agents from water is known in the art, and utilized mainly for environmental purposes, as the anti freeze agents often are considered as pollutants. Large scale systems for separating anti freeze agents from water are frequently found in airports and used to take care of the large amount of anti freeze substances used then defrosting aircrafts. U.S. Pat. No. 5,626,770 describes a system for taking care of the coolant from vehicles, by the use of a series of filters. The purpose being the same as the airport systems.
TABLE 1TemperatureFreezingof mediumpointTransferTransferName(° C.)(° C.)(W/m2K)(%)Temper (salt dissolved in0−101768.60water)Ethylene glycol - water0−101514.8−14Ethanol - water0−101293.4−27Propylene glycol - water0−101248.9−29Temper (salt dissolved in0−301455.40water)Ethylene glycol - water0−301004.8−43Ethanol - water0−30898.2−49Propylene glycol - water0−30666.6−62Temper (salt dissolved in0−101768.60water)Temper (salt dissolved in0−301455.4−18water)Ethylene glycol - water0−101514.80Ethylene glycol - water0−301004.8−34Ethanol - water0−101293.40Ethanol - water0−30898.2−31Propylene glycol - water0−101248.90Propylene glycol - water0−30666.6−47